1. Field of the Invention
This invention relates to welding apparatus and more particularly to welding apparatus adapted for welding a sleeve within a curved or deformed tube using a laser beam.
2. Description of the Prior Art
In tube-type heat exchangers, a first working fluid flows internally through a bundle of tubes, a second working fluid flows externally of the tubes and heat exchange occurs between the two fluids through the tube walls. Occasionally a tube may become defective due to corrosion or other known failure mechanisms so that a leak occurs which allows the two working fluids to mingle. When a leak occurs, not only is heat exchange efficiency diminished, co-mingling of working fluids can cause damage to the heat exchangers as well as equipment connected to it. Accordingly, when a leak occurs it must be either plugged or repaired. Sometimes the tube is taken out of service by plugging both ends on either side of the leak. While such procedure is effective to stop the leak, it causes a reduction in heat exchange capacity because the tube has been removed from service. Sometimes it is possible to repair the tube but often repairs are difficult to effect.
In a nuclear reactor, tube-type heat exchangers are commonly used as steam generators. Generally the working fluid flowing through the tubes of a nuclear steam generator is radioactive. When a defect occurs in a tube of a nuclear steam generator resulting in a co-mingling of the working fluids, not only does the leak reduce the effective heat exchange capability of the device and endanger equipment coupled thereto, it also creates a radioactive contamination hazard. Therefore, whatever steps are taken to stop the leak must be effective. If a repair can be made, this is preferred because heat exchange capacity is not significantly affected.
There are several methods known in the art for repairing tubes. Many of these methods are not applicable to repair heat exchange tubes because the defective portion of the tube may not be readily accessible. The problem of accessibility is compounded in nuclear steam generators because of the radioactive nature of the environment in which such structures are used.
One method used in the repair of heat exchange tubes in a nuclear steam generator involves the use of a metal sleeve having an outside diameter slightly smaller than the inside diameter of the defective tube. The sleeve is inserted into the defective tube and attached to it in such a way as to bridge the defective area of the tube. The ends of the sleeve are welded to the interior surfaces of the tube to thereby effect the repair.
In U.S. patent application Ser. No. 821,675 entitled "Laser Welding of a Sleeve Within a Tube" filed in the name of William H. Kasner of Jan. 23, 1986, a method of welding a sleeve within a tube of a nuclear steam generator is described. The method involves locating the sleeve within the tube at the weld site, positioning a high power laser remote from the weld site, inserting a welding head apparatus at the end of a laser beam transmitting means into the tube near the weld site and communicating laser energy along the transmitting means to the weld site for application to the internal surfaces of the sleeve for welding the sleeve to the tube.
Related U.S. patent application Ser. No. 821,674 entitled "Laser Welding Head for Sleeve to Tube Welding" filed in the name of Hawkins et al. on Jan. 23, 1986, describes the specific arrangement of the laser welding head which includes a straight cylindrical housing which is adapted to extend axially into a steam generator tube and rotate therein for providing a circumferential weld between the sleeve and the tube.
U.S. patent application Ser. No. 821,673 entitled "Laser Beam Alignment and Transport System" filed Jan. 23, 1986 in the name of Hawkins et al. discloses a method and apparatus for aligning the high power laser beam with the desired welding site at the output of the welding head.
According to the aforementioned United States Patent Applications, laser welding has proved to be an effective alternative to other welding methods because it is fast, accurate and produces a smaller heat affected zone. The various methods and apparatus described in the aforementioned patent applications require that the incident laser beam be aligned with respect to the optical axis of the welding head. If misalignment occurs, the laser beam is misdirected resulting perhaps in an inferior weld and possibly damage to the welding head due to absorption of some of the misdirected laser energy.
Proper laser welding may also be affected if the tube and sleeve have an axis different from the axis of the welding head. For example, the welding head described in the aforementioned patent applications is adapted for use in a straight section of a tube. If the tube has a bent or curved portion, the welding head may not be effective to communicate laser energy to the weld site because of misalignment of the tube and sleeve axes with the axis of the welding head.
FIG. 1 schematically illustrates in a simplified form a welding head 10 similar to the apparatus disclosed in the aforementioned patent applications suitable for welding a sleeve S within a tube T secured within a tube sheet TS of a nuclear steam generator. The welding head has an optical axis A and is adapted to direct the laser beam B (shown in parallel dotted lines) through the cylindrical body 12 which has an inlet 14 and an outlet 16. The welding head 10 projects through tube sheet TS into the tube T and concentric sleeve S that are to be welded together. A lens 18 and mirror 20 serve to focus the laser beam B and direct it through the outlet 16 in the cylindrical body 12 and onto the inner surface of the sleeve S at the weld site W where welding occurs. Means (not shown) is provided to rotate the cylindrical body 12 of the welding head 10 about the optical axis A (shown by the centerline) thereby causing a circumferential weld to be made between the inner surface of the tube T and the outer surface of the sleeve S whereby the sleeve is firmly attached to the tube.
There are conditions under which the quality of laser welding performed by the laser welding head 10 described above may be adversely affected. One condition occurs if the axis a.sub.b of the incident laser beam B is significantly misaligned relative to the optical axis A of the welding head 10 as shown in FIG. 2. In such case the laser beam B is projected towards the cylindrical body 12 at an angle q such that the beam strikes the inside wall 22 of the cylindrical body 12 resulting in scattering and absorption losses. Thus, the laser energy is only partially collected and focused by the lens 18 and mirror 20 resulting in the loss of power delivered to the weld site W, and possibly an inferior weld. Also, it is possible to damage the lens 18, mirror 20 and associated mounting structures (not shown) due to scattering and absorption. It is also possible to damage the inside walls 22 of the welding head 10 due to misdirected laser energy.
Another condition that can affect proper laser welding occurs if the tube T and sleeve S to be welded are significantly bend or deformed as shown in FIG. 3 in somewhat exaggerated form for purposes of illustration. In such case, the optical axis A of the welding head 10 is different from the central axis C of the tube T and sleeve S configuration. Thus it may not be possible to insert the welding head 10 sufficiently into the sleeve S to reach the weld site W. Even if it is possible for the weld head outlet 16 to be aligned with the weld site W, the welding head 10 may be too close to the weld site to effect a proper weld. Less severe deformation may permit the welding head 10 to be inserted but may restrict the free rotation of the welding head thereby resulting in damage to the apparatus and possibly an inferior weld.
The present invention as hereinafter described provides means to overcome both of the aforementioned problems. In addition, the improved welding head of the present invention permits the inclusion of a TV viewing system into the welding head assembly thereby providing for in situ inspection of the welds.